It has been shown that the existing definitions of the efficiency of hydraulic jump do not furnish a correct picture of the physical phenomenon. A more rational definition of the efficiency of hydraulic jump on a horizontal floor in a rectangular channel is given by the ratio of the energy actually dissipated in the jump to that required to be dissipated. The jump efficiency increases rapidly with increasing Froude number and it becomes asymptotic to the 100% line at a Froude number of about four. The scouring energy of the flow downstream of the jump also decreases with increasing Froude number. Experimental data have been presented which show that scour below hydraulic structures provided with jump type stilling basins in which the Froude number of the incoming flow is less than 4.5 can be reduced considerably by the provision of floor blocks.